1. Field of the Invention
The present invention related to a sensor for measuring the moisture content and salinity of a material. More specifically, the present invention relates to a sensor for measuring the moisture content and salinity of soil.
2. Description of the Related Art
The prior art discusses several soil monitoring sensors.
There have been several sensor approaches to measuring the moisture and salinity of soil. One such approach is described by U.S. Pat. No. 5,479,104 (the '104 patent). The sensor of the '104 patent uses one oscillator and one frequency, a bridge type scheme with two resistors functioning as reference bridge elements, and three AC meters to measure the unknown capacitance of the soil (Cs) and resistance of the soil (Rs), where the capacitance can be directly related to soil moisture and the resistance to salinity. The use of such a sensor requires the following steps: powering the sensor (which turns on the oscillator); reading the three AC meters; performing numerous calculations to determine Cs and Rs; applying calibration equations to convert Cs to soil moisture measurements and Rs to salinity measurements; and turning off the sensor.
The difficulty with such a sensor scheme is that the calculations to determine Cs and Rs are very complex relations of two ratios: (1) AC Meter 3/AC Meter 1 and (2) AC Meter 2/AC Meter 1. These calculations are particularly complex in actual systems with higher order effects present, thus requiring the use of complex number equations.
A further difficulty is that AC meters that operate at such high frequencies are difficult and expensive. This problem is often addressed by using simple diode detectors in place of standard AC meters. Diode detectors, however, become highly non-linear when the AC voltage lowers to around 0.2 volts such that it becomes difficult to relate the DC meter voltage reading to the actual AC voltage level being measured. At such voltages, the detectors also become increasingly sensitive to variations in the components used. Further, the diode detectors display a significant temperature dependence that requires correction.
Referring to the '104 patent, the basic scheme is shown in FIG. 2 of the '104 patent. To begin with, in the '104 patent, one frequency is used, 50 MHz in the built version of that sensor. It employs a bridge type scheme with R1 and R2 as known reference bridge elements, along with three AC meters to measure the unknown capacitance and resistance of the soil (element 24). The capacitance can be directly related to soil moisture and the resistance to salinity. The calculations to determine Cs and Rs are very complex (particularly in actual systems with higher order effects present) relations of two ratios—(AC Meter 3/AC Meter 1) and (AC Meter 2/AC Meter 1). Essentially there are two measured ratios which are then used to solve for two unknowns—Cs, Rs. The simpler first order calculations are shown in the patent (column 6, near line 50).
The actual steps in the measurement process are as follows: power the sensor (which activates the oscillator), read the three AC meters, perform the calculations to obtain the Cs value to soil moisture and Rs value to salinity, and then deactivate the sensor.
Because AC meters that operate at these high frequencies are difficult and expensive, simple diode detectors are used as a proxy for AC meters. These are shown in FIG. 3 of the '104 patent. The output of these meters is a DC voltage read by the DC Meters. The DC meters are inexpensive, but have problems. Once the AC voltage gets low (around 0.2 volts or so), the detectors stop working well. Even at AC voltages around 0.3V, the detectors become highly non-linear (i.e. it is not easy to relate the DC Meter to the actual AC voltage level being measured) and the detectors become increasingly sensitive to component variations (the diode, resistor, and capacitor used). In addition, these detectors also display a significant temperature dependence that needs to be corrected for in the measurement. At the end of the line, the calculations needed to correct for detector temperature effects and non-linearity, as well as calculating Cs and Rs including the higher order effects, which becomes very complicated.
To provide accurate moisture and salinity measurements, therefore, current sensors must calculate capacitances and resistances, including higher order effects, using very complicated complex number equations that can also correct detector temperature effects and detector non-linearity.
Thus, there is need for a sensor to measure the capacitance and resistance of soil that uses simpler calculations, provides improved detector performance, and, ultimately, provides more accurate moisture and salinity measurements.